High-strength galvanized steel sheets for automobile and other components are required to have excellent formability in addition to high strength for the character of their applications.
Recently, high strength steel sheets have been demanded in manufacturing of automobile bodies as measures to improve fuel efficiency by reducing the weight of car bodies while ensuring crashworthiness and, thus, have increasingly been used. Although their main field had been simple processing, applying this type of steel sheet to components having complicated shapes has recently been considered.
In general, however, formability of steel sheets decreases as their strength increases. Thus, one of the most serious problems with the use of high strength steel sheets is fracturing during press molding. There is thus demand for stretch flangeability and other formability characteristics to be improved as required by the shape of the component. For steels having a strength as high as 980 MPa or more, bendability is also important, particularly because parts made from this type of steel are more often processed by bending.
Furthermore, excellent weldability is also essential in addition to formability because shaped steel sheets are subjected to resistance spot welding during assembly.
Moreover, components having a reduced thickness should have better bending-fatigue properties than ever in some areas thereof.
As solutions to satisfy these requirements, for example, Japanese Unexamined Patent Application Publication Nos. 2004-232011, 2002-256386, 2002-317245 and 2005-105367, Japanese Patent Nos. 3263143 and 3596316 and Japanese Unexamined Patent Application Publication Nos. 2001-11538 and 2006-63360 disclose methods of obtaining a high-formability and high strength galvanized steel sheet by limiting the composition and microstructure of steel and optimizing hot-rolling and annealing conditions.
Of the patent publications, JP '011 discloses a TS 980 MPa-class steel containing large amounts of C and Si, but does not regard stretch flangeability and bendability.
JP '386, JP '245 and JP '367 disclose steels containing Cr, but also pay no attention to flangeability and bendability.
In JP '143, JP '316 and JP '538, which mention hole expansion ratio λ, a measure for evaluating stretch flangeability, the tensile strength (TS) is lower than 980 MPa. Furthermore, these publications make no mention of bendability and fatigue properties.
JP '360 mentions improvement of bendability by the addition of Ti and improvement of notch fatigue properties by a reduced ferrite grain size, but does not discuss stretch flangeability, weldability, or bending fatigue. Notch fatigue properties are an evaluation measure of fatigue breakdowns occurring from perforations for bolting and attaching fixtures. Bending fatigue properties are fatigue properties of the base material itself, which is the main ingredient of finished components. These two kinds of properties are quite different in that for perforations, the status of cracks that occur at perforation greatly contributes to the fatigue properties, the fatigue in smooth areas greatly depends on the microstructure and composition of the base material.
It could therefore be helpful to provide a high strength galvanized steel sheet having excellent formability, weldability, and fatigue properties and a tensile strength as high as TS 980 MPa together with an advantageous method for manufacturing it.